1. Field of the Invention
The present invention relates to a process for producing toner particles to be used for visualizing an electrostatic latent image in an image-forming method such as an electrophotography, an electrostatic recording method, a magnetic recording method, or a toner-jet method.
2. Description of the Related Art
Various kinds of methods of electrophotography have been known in the art as described in U.S. Pat. No. 2,297,691 and so on. Typically, the electrophotography forms an electric latent image on a photo conductor by various kinds of means using a photoconductive material, develops the latent image using a toner, and transfers the toner image on a recording material such as a sheet of paper as necessary, followed by fixing the toner image on the recording material by the application of heat and pressure, vapor of solvent, or the like to obtain a copied product. Conventionally, various kinds of methods have been proposed as methods for developing an image using toner or methods for fixing toner images. Of those, methods suitable for the respective image-forming methods have been used.
In recent years, a high speed printing and a high image quality have been desired for electrophotography.
In general, as a process for producing toner, a pulverizing process has been known in the art. The pulverizing process includes the steps of melting and mixing a colorant such as dye or pigments and an additive such as a charging-control agent in a thermoplastic resin, and dispersing the mixture uniformly, followed by pulverizing with a pulverizer and classifying with a classifier to obtain toner with a desired particle size.
However, the production of toner obtained by the pulverizing process is restricted when a releasing agent such as wax is added in the toner. That is, the restrictions for obtaining a sufficient degree of dispersibility of the releasing agent include that (i) at the temperature for kneading with a resin, there is a need to keep the viscosity of a kneaded material at a certain degree, (ii) the content of a releasing agent is about 5 parts by mass or less per 100 parts by mass of toner, and so on. Because of those restrictions, the fixing property of toner produced by the pulverizing process is limited.
In the pulverizing process, further, it is not easy to attain a completely uniform dispersion of solid fine particles of colorant and so on in the resin. Depending on the degree of dispersion, the composition of toner becomes unbalanced and the developing characteristics of toner may be varied. Further, in general, the resolution of an image formed by the toner, the uniformity of a solid portion in the image, the reproducibility of gradation, and so on are largely dependent on the characteristics of toner, in particular on the particle size of toner. That is, the smaller the particle size of toner, the higher the quality of an obtained image. Therefore, toner having a small particle size is used in most of recent printers, high-quality copying machines, and so on. However, in making the toner particles small by a pulverizing process, a volume average particle size of about 5.0 μm is the limit due to the ability of a pulverizer.
Further, in this pulverizing process, a step of classifying the resulting toner is indispensable for obtaining predetermined particle size and particle size distribution. Therefore, since this step generates fine particles and coarse particles in addition to the toner having a predetermined particle size, various contrivances have been made with respect to the producing process for achieving reutilization thereof.
Further, the above coarse particles are pulverized again in the producing step, so that the coarse particles are made into fine particles. On the other hand, as described in JP-A-5-34976, and so on, the fine particles are conventionally reutilized recycling a predetermined amount of the powder in the step of mixing raw materials, due to considerations regarding the environment, production costs, and the like. However, this process is not preferable because of the following reasons. That is, at the time of melting and kneading the above fine particles again by a kneading machine, the resin molecules in the fine particles are cleaved again to decrease the molecular weight of the resin component. At the time of fixing the toner on the paper, therefore, the deterioration of the fixing performance such as hot offset occurs. In addition, as the mechanical strength of toner decreases, the durability of performance of the toner becomes deteriorated.
For improving those problems in the conventional methods, in JP-A-8-69126, and soon, various contrivances have been proposed, such as processing fine particles before charging them into the process for kneading. The reutilization of toner by charging fine particles into the process for kneading is broadly performed as a well-known technique in the art for providing an economical and productive process for producing toner.
In contrast, there is also proposed a process for producing toner in which a polymerizable monomer composition containing at least a polymerizable monomer is suspended and polymerized while simultaneously obtaining toner particles (JP-B-36-10231). Hereinafter, toner obtained by such a process will be referred to as “polymerized toner”. This suspension polymerization is a process in which a polymerizable monomer and a colorant (optionally, also a polymerization initiator, a crosslinking agent, and other additives) are uniformly dissolved or dispersed to obtain a polymerizable monomer composition, followed by dispersing the polymerizable monomer composition in a continuous phase (e.g., an aqueous phase) containing a dispersion stabilizer using an appropriate stirrer while allowing a polymerization reaction to occur at the same time, thus obtaining toner particles having a desired particle size. Attention has been recently particularly focused on this process because it has various advantages without having any of the restrictions associated with the pulverizing process described above.
In other words, with regard to the content of a releasing agent and the dispersibility thereof, the above polymerized toner can contain a certain amount of the releasing agent in the inside of a toner particle. Therefore, it is possible to increase the content of the releasing agent in this process, as compared with the pulverizing process. In this case, further, the dispersibility of the releasing agent can be simultaneously satisfied. Further, a colorant can be also uniformly dissolved or dispersed in the polymerizable monomer together with other additives, so that there is no particular problem regarding the dispersibility of the colorant. Depending on the conditions of dispersion and granulation, desired particle size and particle size distribution can be controlled, so that there is another advantage in that the polymerized toner can be used for the production of small-sized toner particles.
However, the polymerized toner has the following disadvantages to be solved.
That is, regarding the polymerized toner, the aggregation of particles occurs during a polymerization reaction depending on the reaction conditions and the formulation of toner. As a result, aggregates of polymerized particles are adhered on the wall surface of a reaction vessel, a stirring blade, and so on. In addition, it is not easy to completely exclude the mixture of these coarse particles even under the producing conditions in which the particle size distribution width of toner particles is narrowed by the steps of dispersion and granulation and the aggregation of particles is prevented as much as possible by various kinds of techniques.
On the other hand, a dispersant is used for narrowing the particle size distribution width of toner particles. Depending on the concentration of the dispersant in a water phase and the conditions for adding the dispersant to be used, a polymerization reaction is accompanied even in the water phase, so that ultra-fine particles of 0.1 to 1 μm in diameter or less may be generated. The presence of such ultra-fine particles causes problems with respect to the image characteristics of toner (i.e., the density of a solid image, the uniformity of an image density, fogging, and so on) because distribution of the colorant or the like is not uniform in the ultra-fine particles. Moreover, in the case of toner in which the ultra-fine particles are adhered on the surface of toner particles, similar problems occur in terms of the image characteristics of toner because the property of toner such as flowability and charging controllability are changed.
With the trend toward a higher-quality image of electrophotograph, further narrowing of the particle size distribution is required of polymerized toner. In the current techniques, even if granulation conditions are optimized, it is often necessary to decrease the percentages of fine particles of 4 μm or less in particle size and coarse particles of 10 μm or more in particle size in the toner.
In addition, from the different viewpoint, the polymerized toner is typically designed as particles where each particle has a core shell structure comprised of at least two layers by incorporating a releasing agent, a low-energy fixing component, and so on in the particle. In this kind of the polymerized toner, when toner particles that are beyond predetermined ranges of particle size distribution and particle size distribution width are generated in some form or another, the reutilization of toner cannot be simply attained just as in the case with the toner obtained by the pulverizing process. This problem is an important issue to be solved in view of the yield of toner.
On this point, JP-A-10-301330 proposes dissolving a kneaded material containing a THF insoluble resin component other than the desired toner particles in a polymerizable monomer and then recycling it. In this proposal, the soluble components in non-desired toner components are only used. In other words, the insoluble components cannot be used, so that a recycling rate of the toner is not 100%. Consequently, there is a need to further improve the recycling rate of the toner.